The alveoli and distal airways of the lung are lined with an acellular layer of material which is essential for the maintenance of normal pulmonary functions such as gas exchange. The composition and origins of the acellular lining are being investigated. Current attention has been directed towards: (1) the biosynthesis and secretion of pulmonary surfactant and (2) the origins of several extracellular enzymes present in the lining. The objectives of this investigation are as follows: (1) to elucidate the composition of the acellular lining; (2) to identify processes by which these components arise in the acellular lining; (3) to identify the subcellular processes involved in the formation of acellular lining components synthesized by the lungs; (4) to identify variations in the acellular lining of diseased and damaged lungs.